Oil producers utilize different means to produce steam for injection into the oil bearing formation. The steam that is injected into the geologic formation condenses by direct contact heat exchange, thus heating the oil and reducing its viscosity. The condensed steam and oil are collected in the producing well and pumped to the surface. This oil/water mixture, once the oil has been separated from it, is what is referred to as ‘produced water’ in the oil industry.
Since water can comprise up to 90% of every barrel of oil/water mixture removed from the formation, the recovery and reuse of the water is necessary to control the cost of the operation and to minimize the environmental impact of consuming raw fresh water and subsequently generating wastewater for disposal. Once the decision to recover water is made, then treatment of those produced waters is required to reduce the scaling and/or organic fouling tendency of the water. This treatment generally requires the removal of the hardness and other ions present in the stream, preferably to near zero. As is understood in the art, the ‘hardness’ causing ions are the combined calcium and magnesium salts in the water to be used in steam generation equipment and is typically expressed as parts per million (ppm) although other terms can be used. While silica is not considered as adding to the hardness value, its presence can also lead to scaling problems if present in other than minimal amounts.
The traditional method for generation of steam in enhanced oil recovery is to utilize a once-through steam generator (OTSG) in which steam is generated from a treated feedwater through tubes heated by gas or oil burners. The OTSG feedwater can have a total dissolved solids concentration as high as 8,000 ppm, but requires a hardness level that is 0.5 ppm (as CaCO3) or less. This method produces a low quality or wet steam, which is approximately 80% vapor and 20% liquid, at pressures ranging from 250 pounds per square inch gauge (psig) up to 2400 psig. This 80% quality steam either directly injected into the formation or in same cases the 80% vapor is separated from the 20% water and then the vapor is injected into the formation. Either a portion or all of the 20% blowdown is disposed as a wastewater.
Another method that has been used to obtain the high quality steam requirement is using a water tube boiler instead of the OTSG to generate steam. The water tube boiler, however, requires an even greater amount of feedwater pretreatment than the OTSG to ensure problem free operation. The lime soda softening, media filter, and polishing WAC are replaced by a mechanical vapor compressor evaporator (MVC). A very large electrical infrastructure is required to supply power to the MVC evaporator compressors and power consumption is high due to MVC evaporator compressor. The concentrate from the evaporator in the case of high pH operation is difficult to process, requiring expensive crystallizers and dryers or expensive offsite disposal.